The present invention relates to a circuit and to a method for extending the range of the output voltage of an integrator circuit beyond its supply voltage. More particularly, the present invention relates to a circuit of this type associated with an integrator circuit used in automotive applications, and, more specifically, in detecting knocking in internal combustion engines.
In a system for detecting knocking in an internal combustion engine, one or more wide-band accelerometric knock sensors are provided, and advantageously are disposed on the engine block in the vicinity of the cylinders. These sensors register variations in pressure on the cylinder walls and translate them into electrical signals which are processed in a control unit to distinguish the pressure contributions due to knocking from those relating to operation with correct combustion.
During this processing, the electrical signal coming from the sensor is amplified and filtered, and after being rectified, is sent to an integration stage which outputs a voltage signal. This voltage signal is proportional to the energy of the initial electrical signal, is within the filtering band, and is proportional to the integration period.
At the end of the integration period, the value of the voltage signal reached by an integrator circuit of the integration stage is stored, for example, in a sample/hold circuit and made available as an output to further processing stages. These further processing stages are arranged to identify the occurrence of knocking from the value of this signal and to provide feedback control to a system controlling ignition in the engine.
It can easily be understood that the value of the voltage signal output by the integrator circuit may reach high levels if the integration time is long. Conventional integrator circuits formed with operational amplifiers and capacitive feedback components have a maximum limit for their output voltage, which may increase or decrease monotonically within the integration time period. This limit cannot be passed and is determined by the supply voltage supplied to the circuit, or by the supply voltages if there are two, that is, one positive and one negative.
When an operational amplifier is required to have an output voltage close to or greater than this limit, it ceases to operate linearly and reaches a saturation condition in which the voltage no longer increases (decreases) as the integration time passes. Instead, the voltage adopts a maximum (minimum) limit value which is substantially constant and is within the limits imposed by the supply voltage.
The approaches according to the prior art, which are referred to in the technical literature as xe2x80x9crail-to-railxe2x80x9d circuits, do not provide for these limits to be exceeded, but only to be approached as closely as possible.
An object of the present invention is to provide a system which enables the range of the output voltage of an integrator circuit to be artificially extended beyond the limits imposed by the supply voltage.
A circuit for extending an output voltage range of an integrator circuit that receives an input signal for providing an output signal having a voltage that develops monotonically within a range of values is provided. The circuit preferably comprises a control circuit connected to the integrator circuit for control thereof so that the integration circuit starts a subsequent integration of the input signal within an integration time period each time a voltage of the output signal reaches a limit of the range of values.
A counter is preferably connected to the control circuit for counting a number of times the voltage of the output signal covers the limit of the range of values. An actual voltage of the output signal at the end of the integration time period is calculated based upon a final voltage of the output signal at the end of the integration time period, and from the number of times the output signal covers the limit of the range of values.
In summary, the present invention is based on the principle of monitoring the development of the voltage signal generated by an integrator circuit according to the prior art and resetting the circuit (or, in an alternative embodiment, reversing the characteristic slope of the output signal) each time its output voltage reaches a predetermined limit close to the saturation condition.
This is combined with the step of memorizing the number of occasions on which these interventions have occurred by using a counter which is connected to the integrator circuit, and which is incremented each time the integrator is reset (or the slope of the output signal is reversed).
At the end of the predetermined integration period, the content of the counter will thus indicate how many times the voltage signal generated by the integrator has covered the entire range naturally available during its increasing or decreasing development. This will enable the actual voltage value of the signal resulting from the integration to be calculated by a simple mathematical operation from the reading of the counter and from the signal currently present at the output of the integrator, as will be described further in the following examples.
The embodiment according to the present invention thus enables a substantially unlimited, although fictitious, output voltage range to be provided in an integrator circuit. In a circuit of the type used, for example, in control systems for detecting the degree of knocking in internal combustion engines, this enables simpler control systems to be produced. These control systems are advantageously produced which operate from a single voltage supply (for example, 5V) both for the active elements of the integrator circuit and for the logic circuits, and any micro-controllers present in the engine electronic control unit.
A further advantage is that the integration period can be extended at will and more efficient engine control algorithms can be established. The greater efficiency achieved enables an engine of the same type to have lower consumption and greater power than with current approaches.